With the most conventional multimeters, a voltage measurement is quite easy. However, to measure a current in a circuit, one side of the component should be disconnected and a current meter is connected between the 2 open terminals. To measure a resistance value in a circuit, one side of the resistor should be disconnected from the circuit and an Ohmmeter is connected across the resistor, and measures the resistance. Disconnecting one side of a component means unsoldering the component. After finishing the measurement, the component must be connected back by soldering it to original state. There is a current meter, called clamp meter, which can be used to measure a current without opening a circuit and connecting the meter between the open terminals. However, the meter can be used to measure very high current, most cases in several amps in an electrical circuit. It can not be used in an electronic circuit where a current value is normally miliamp range. To avoid those inconveniences disconnecting a component from a circuit and reconnecting the component back in the circuit, the present invention is developed for a circuit where 2 or more resistors are connected in series. The present invention also can be applied to analyze a transistor circuit.
This invention is to facilitate measurements of voltage, current, and resistance in an electronic series circuit. All 3 values are measured simultaneously. Therefore, the 3 functions together are defined as a VOM function. It is not necessary to disconnect any side of a component to measure a current or a resistance in a series resistors circuit. A voltage is measured across a component in conventional way by a typical voltmeter function. By default, all switches connected in series with all parallel range resistors of the VOM function are open state. To measure a resistance without disconnecting the resistor, firstly, measure V+ (supply voltage of the circuit under test), and measure the voltage across the load resistor, designate this voltage as VL, and put the highest parallel range resistor (RP) in parallel with the load resistor by activating the appropriate switch, then measure the voltage appearing across 2 parallel connected components, load resistor and the parallel resistor, define this voltage as VP. If the VP is lower than the predetermined percentage of VL, calculate the value of RL (load resistance) by using the values of V+, VL, VP, and RP. If the Vp is higher than the predetermined percentage of VL, then open the highest parallel resistor by deactivating the series switch, and connect the next highest range resistor in parallel with the load resistor by activating the appropriate switch, and continue the process same as performed with the highest resistor. Repeat changing the parallel resistors until it finds an appropriate range at which Vp is lower than the predetermined percentage of VL. Then, compute the value of RL, by using VL, VP, V+, and RP. After RL is found, calculate the current of the load resistor (IL) by the equation, IL=VL/RL.
For the circuit where a single resistor is connected directly across a power supply, the above theory can not be applied, and conventional method must be used for the measurements of a current and a resistance. To measure a resistance for the one resistor circuit, one side of the resistor should be disconnected from the circuit and measure the resistance.
For the measurement of a current for one resistor connected across a power supply, one side of the component should be disconnected and a current meter is connected between the 2 open terminals, and measure the current in conventional way. If 2 or more resistors connected in parallel between 2 nodes and they are connected in series with another resistor, the same principle of the present invention can be applied. However, this approach measures a total resistance and a total current between the 2 nodes. To measure individual branch current of a parallel circuit, the conventional method of current measurement and resistance measurement method should be applied.